Various techniques have been proposed as data distribution techniques. For example, on the present Internet, a protocol based on TCP/IP (Transmission Control Protocol/Internet Protocol) such as HTTP (Hyper Text Transfer Protocol) is employed. In accordance with the TCP/IP, calling to the data transmission side is carried out on the receiving side where distribution of data is received, and connection between the transmission side and the receiving side is established every time data transmission/reception is carried out. Therefore, highly reliable distribution of data can be carried out. However, the load on the transmission side and network is increased, and therefore, in some cases it is difficult to carry out efficient data distribution.
Specifically, if the number of terminals which receive provided data is increased and access to the server providing data is concentrated, a very large load is applied on the server and network, and it takes quite a long time to obtain data after requesting the data.
Thus, there is recently proposed a technique of DRP (Distribution and Replication Protocol) in W3C (World Wide Web Consortium). In accordance with the DRP, in the case where data is updated in one database, only differential information between the data before updating and the updated data is individually distributed to other databases dispersed in a number of regions, instead of distributing the updated data itself. Then, in the database where the distribution of the differential information is received, the data before updating is updated on the basis of the differential information.
Therefore, since the differential information is transmitted in the data distribution method using the DRP, the quantity of information flowing on the network can be reduced in comparison with the case where the data itself is transmitted. However, if access from a large number of terminals is concentrated, the load on the server and network is increased, making it difficult to carry out efficient data distribution.
In the data distribution using the TCP/IP or DRP as described above, connection between the transmission side and the receiving side is established so that data is transmitted and received. On the other hand, there is a data distribution method using UDP (User Datagram Protocol) such as IP (Internet Protocol) multicast for carrying out connectionless transmission/reception of data. Since the data distribution using the UDP is connectionless data distribution, the reliability of data is lowered in comparison with the data distribution using the TCP/IP or the like based on the established connection, but the load on the transmission side and network can be reduced.
However, even in the case of the UDP, data must be distributed individually to each of many terminals as long as the present Internet is used. Therefore, even when data distribution from the server is automatically carried out, the increase in the number of terminals increases the load on the server and network, making it difficult to carry out efficient data distribution.
In any of the above-described data distribution methods, the increase in the load on the server and network due to the increase in the number of terminals cannot be avoided in the case where data distribution is carried out via the Internet connected by physical lines such as cables.
Thus, there is proposed a method for data distribution using a satellite circuit or a ground-wave program circuit of a broad frequency band which enables simultaneous multiple address over a wide region. In this case, the load on the server and network is not influenced by the increase in the number of terminals.
Meanwhile, in simultaneous data distribution using a satellite circuit or the like, the same data is distributed to each user. However, when certain data is distributed, not all users want that data. That is, when data distribution to an extremely large number of users is carried out, it is typical that some users want that data while others do not. Since data to be distributed is generally determined on the transmission side, distribution of data demanded by a small number of users might be preferentially carried out rather than distribution of data demanded by a large number of users. Such a case is not preferred in consideration of efficient data distribution.
Moreover, in simultaneous multiple address of data using a satellite circuit, a certain user who immediately needs certain data might not be provided with that data immediately. It is difficult to deal with such a case.